This invention relates to a method for erasing recording in a PHB memory and more in detail to a method for erasing (recording permitting to erase) rapidly data or information stored in a PHB memory.
Recently the method for recording and reproducing information by means of a light beam is considered to be extremely promising as future information recording means and it is desired to realize increase in recording density, possibility of rewrite, increase in and output speed, etc. as problems to be solved for use in practice. In order to solve the problems described above, research thereon such as new optical recording mediums, multiplication of the wavelength using a multilayered structure, etc. is carried out actively. Among them there is known the PHB (photochemical hole burning) memory utilizing hole burning phenomena, which will be explained later.(U.S. Pat. No. 4,101,976). The hole burning phenomena described above are scientifically interesting and known as one of high resolutional power spectroscopic methods. The PHB memory stated above utilizing these phenomena is a multiple wavelength optical memory, by means of which it is expected to increase the recording density as remarkably as about 1000 times with respect to a prior art memory. The principle therefor will be explained below.
When photochemically active molecules are embedded in a low temperature solid matrix, its absorption spectrum reflects delicate differences of the environment, in which the photochemically active molecules are embedded, and exhibits broad inhomogeneous absorption spectrum as indicated in FIG. 1A. When the molecules stated above are irradiated with a laser light beam having a narrow line width specified wavelength .lambda..sub.1 within the spectrum stated above and a high intensity, only the molecules resonating with the wavelength .lambda..sub.1 absorb the laser light and pass to a metastable state, which is different from the initial ground state, via an excited state. It is supposed that the metastable state mentioned above can exist due to (i) displacement of a proton within a molecule due to light absorption, (ii) change in alignment of molecules in a matrix due to light absorption, (iii) photodissociation of a molecule due to light absorption, etc. When molecules are held in the metastable state described above, it is observed that the absorption intensity is reduced only at a wavelength .lambda..sub.1 in the irradiated laser light and a hole having a narrow line width is formed in the absorption spectrum, as indicated in FIG. 1B. The phenomenon, by which a hole having a narrow line width is thus formed in the absorption spectrum, when molecules are irradiated with light, is called hole burning. Further, when the molecules are irradiated with laser light while varying its wavelength successively to .lambda..sub.2, .lambda..sub.3, . . . , a hole is formed at each of positions corresponding to the wavelengths .lambda..sub.2, .lambda..sub.3, . . . described above, as indicated in FIG. 1C, and in this way a multiple wavelength optical memory can be formed.
Since the fundamental principle of the PHB memory is based on delicate variations induced by light in the state of the molecules, it is necessary to pay sufficient attention to the stability of the hole in the absorption spectrum in order to hold the memory. In fact, it is known that when the temperature of the recording medium is raised to 30 to 50 K, the hole recorded by the light irradiation cannot be held stably and the hole disappears. For this reason, for the PHB memory the whole recording medium must be conserved in a cryogenic state at the liquid helium temperature of 4.2 K, at which thermal fluctuations of molecules are small, in order to prevent disappearance of the memory stated above.
As described above, research is carried out on write-in of information and holding the memory at the cryogenic temperature for the PHB memory. However the method for erasing stored information is not studied at all and no PHB memories, for which it is possible to write-in and erase information, have been yet realized. In particular, speaking from the practical point of view, it is inevitable to erase selectively a part of the stored information and nevertheless such partial erase of information has not yet been resolved.